In joining metal parts by various known soldering techniques, a soldering flux is often required. This soldering flux performs several functions. First, it promotes the formation of a sound joint by preventing the formation of oxide films during heating and also serves to remove any oxide film already present. In addition to breaking up the oxide films on the surface of the metal, and protecting the fresh metal surface from reoxidation, the soldering flux must be capable of forming a uniform coating of proper thickness on the metal member and also reduce the surface tension of the molten solder, thereby facilitating wetting, flow, and capillarity of the solder over the prepared surface. Another desirable property in a soldering flux is that it must be capable of being readily displaced from the solid metal by the liquid solder without leaving a corrosive residue and without attacking, more than superficially the solid metal. In order to achieve this ready displacement without concurrent deposit of flux residue, the flux should be fluid below soldering temperature, and react at a temperature below the solder's liquidus.
Several reaction salt-type soldering fluxes have been heretofore proposed to obviate these difficulties. For example soldering fluxes containing zinc chloride, ammonium chloride, and an additional alkali metal halide or ammonium halide other than chloride have been used, either in dry form or mixed with a suitable solvent. However, these fluxes have been found to either leave an undesirable corrosive halide residue on the metal surface requiring cleaning with cleaning solvents such as chlorofluorcarbon (CFC) solvents, or have resulted in a roughened surface on the joined members. The soldered joints are also found to have poor corrosion resistance evidenced by the large increase in electrical resistance upon exposure to moist atmospheres.
Japanese 62[1987]-16898 discloses a type of flux for soldering characterized in that it contains stannous fluoride as the activator. The stannous fluoride is used in combination with well known binders such as rosin, rosin phenol, polyethylene glycol, etc. It has been found that metal substrates that have been treated with fluxes containing stannous fluoride and for e.g. rosin have to be used immediately. The rosin forms a tacky coating on the substrate which makes storage of the fluxed metal substrates difficult.
Japanese 64[1989]-40197 discloses a flux consisting of 1-20 weight % of stannous fluoride and the balance of potassium fluoroaluminate complex. The inventor claims use of greater than 20% of the stannous fluoride results in the brazing properties being degraded.
It has been found that using the soldering process of the invention wherein stannous fluoride is used prior to soldering the above disadvantages can be overcome. Metal substrates can be treated and stored without any difficulty until they have to be soldered. Still further, soldering of a metal substrate using stannous fluoride results in a soldered substrate having low ionic contamination and does not require a cleaning operation.